1. Field of the Invention
This invention in general relates to photography and in particular to photoelectric type radiometers having preferred spectral sensitivity characteristics which are particularly useful in photographic exposure control applications.
2. Description of the Prior Art
What a photographer would like to achieve when he takes a picture is to have a permanent record which faithfully reproduces his subject or scene or at least those aspects of his subject or scene which he considers most important. He judges the quality of the photographic reproduction by visually comparing the print with the original scene in the case of self-developing type film where the print is nearly immediately available or with what he remembers the scene to be in the case of more conventional type films. In making this judgment he is comparing the magnitude of the visual sensation created by the photograph under the prevailing viewing conditions with the magnitude of the visual sensation created by the original scene under the actual lighting conditions or what he remembers them to be, i.e., he compares the brightness of various points in the photograph with the brightness of corresponding points in the original scene and thereby forms a subjective opinion about the quality of the reproduction. Exact subjective tone reproduction requires that the brightness of each point in the photograph equal that of the brightness of corresponding points in the original scene. However, as those skilled in the photographic arts know, exact subjective tone reproduction is extraordinarily difficult and inconvenient to achieve because photographs are generally viewed at illumination levels which are significantly less than those under which the photograph was taken-typically about 1/100 that of the original scene. This fact and the fact that most films, as their sensitometric characteristics indicate, have a limited ability to capture the range of tones which typically exist in nature would seem to indicate that a photographer, amateur or professional, could never be satisfied by the photographs that can be produced with the present level of photographic technology. However, this is obviously not the case and the reason for it is that satisfactory subjective tone reproduction can be obtained and will please the photographer if the photographic brightnesses under ordinary viewing conditions are approximately proportional to corresponding scene brightnesses, if the brightness of photographic skin tones approximately equals that of real skin under the prevailing viewing conditions, if the apparent hue and saturation of object colors is mantained relative to the original scene, and if the film reproduces tones corresponding more or less to the range of tones represented by the important objects of the scene.
To assure that the foregoing conditions are more or less satisfied depends, ultimately, on properly exposing the film taking into account the particular scene characteristics, prevailing scene lighting conditions, film characteristics, and camera optical system performance. Given the variety of possible scenes and lighting conditions, proper exposure can only be achieved regularly by understanding the complex interrelationships of the photographic system and some probability estimate of the likelihood of the occurrence of typical scenes. This would include knowing, for example, the most likely distribution and intensities of scene illuminance patterns expected to be photographed, the spectral reflectivity of commonly recurring objects expected to be photographed, the spectral content of likely scene illuminance, and the spectral response and sensitometric characteristics of the film. In currently available amateur camera products, all of these interrelationships are typically automatically correlated for optimum exposure by a camera's automatic exposure control system which commonly utilizes a built-in "averaging" or "center-weighted" type meter for exposure prediction purposes.
One of the most popular known averaging type exposure meters for evaluating scene brightnesses to predict and/or control photographic exposures is the unicell photometer. Typical examples of such unicell photometers for exposure control purposes are shown and described in U.S. Pat. No. 4,040,751 issued to Philip G. Baker, et al., on Aug. 9, 1977, entitled "Unicell Photometer Device;" U.S. Pat. No. 4,102,581 issued to William T. Plummer on July 25, 1978, and entitled "Unicell Photoelectric Photometer;" and U.S. Pat. No. 4,105,300 also issued to William T. Plummer, Aug. 8, 1978, and entitled "Defocused Unicell Photometer with Aspheric Zone."
The disclosures in the aforementioned patents indicate that it is desirable for such unicell photometers to have a nearly precisely photometric response whose spectral sensitivity is substantially identical to the photopic visual response curve of a standard photometric observer according to the Commission Internationale de l'Eclairage, CIE. This is acheived by altering the spectral response characteristics of the photodetector of these photometers, which is preferably a silicon photodiode, through the use of well-known commercially available light attenuating filters that remove most of the light in the IR region of the spectrum from 700 to 1200 nanometers. Thus, these photometers prevent near IR light which is present in the photographic scene from reaching the photometer system of the camera so that the spectral response of the photometer system closely matches that of the photographic film which also is typically sensitive to only light in the visible region of the spectrum. Thus in photographic systems which utilize these types of photometers, the spectral response characteristics of the observer, the photometer and the film are substantially identical.
One reason for matching the spectral response of these types of photometers to the visual response of the human eye is to avoid any exposure variations which may be introduced as a result of differences in the spectral energy content or correlated color temperatures of scene illuminance sources. For example, if the scene illuminance source color temperature is low, say less than 2800.degree. K., then the slightest leak of IR light can result in underexposure as a result of the high IR content of this radiation. High color temperatures, say greater than 7000.degree. K., have excessive blue light content which results in overexposure of film because the photometer has poor blue response and cannot be precisely photometric under this spectrally unbalanced lighting situation. Although these unicell photometers avoid such variations and, as well, are designed to optimize exposure performance over the widest possible range of anticipated scene conditions, further complications in exposure can arise that are less dependent on source color temperature variations. These complications arise when the reflectivities of different objects in photographic scenes exhibit widely different values. Under the situation, for example, where the facial skin of a subject reflects more light than the surrounding clothing or other objects, a scene-averaged photometric response will favor correct exposure of the darker objects, rendering the facial skin (or any other high reflectivity object) substantially overexposed. On the other hand, if the scene consists largely of high reflectivity objects, the same facial skin may be underexposed since, again, the photometer response is to the scene-averaged high reflectivity.
These phenomena apparently occur because the silicon photodetector, once protected from receiving any IR radiation, responds only to the different reflectivities of the scene which are in the visible spectral region. This means that the photometer response is dependent on its acceptance angle or angular response characteristic and this can lead to poor exposure control in complex reflected light situations.
Therefore, there is still a need for an improved unicell type radiation measuring device which will tend to alleviate the aforementioned exposure control problems in photographic exposure control applications, and it is a primary object of the present invention to provide such a device.
It is another object of this invention to provide an improved radiation measuring device having a preferred relative spectral sensitivity characteristic, both in the visible and near IR regions of the spectrum, by which exposure control capability is improved as a result of advantageously utilizing the reflected IR radiation from commonly occurring photographic scene objects.
Other objects of the invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises the apparatus possessing the construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure.